Celebrities have a well-manicured finger on the pulse of what's trendy, which is why we were surprised to hear that many stars are trading in their fruit-juice fasts and cleanses for old-school diet foods: grapefruit, cottage cheese, rice cakesstuff your mom nibbled on before darting off to Jazzercise.

Case in point: Jessica Simpson and Lauren Conrad are grapefruit fans. To prevent their thighs from developing a curd-like texture, Samantha Harris and Bachelorette star Ashley Hebert crack open the cottage cheese. And Jennifer Hudson, the poster girl for successful dieting, snacks on popcorn.

This shift is not lost on experts, who say there's not only a trend toward retro weight-loss methods, but new research to back up the slimming benefits of many of these foods.

What's old is new againInterestingly, millennialswho were born in the 1980s and '90s and thus were not even alive when these foods dominated the diet world in the '60s and '70sare driving this movement back to the basics. "They yearn for authenticity and are distrustful of gimmicks," says Phil Lempert, food trend expert and founder of supermarketguru.com. So retro foodswhich are often minimally processed, easy to eat, and so low in calories and fat they're almost guaranteed to workappeal to them, Lempert adds.

And no matter your age, flashback foods can provide comfort. During uncertain timeslike nowwe revert to things that we remember from childhood and trust will work. "Nostalgic feelings often make people gravitate towards classic diet items," says Susan Albers, PsyD, a psychologist at the Cleveland Clinic Family Health Center and author of Eating Mindfully. "My clients tell me that they eat cabbage soup or low-fat cheese because those are things their mothers used to eat while dieting." (That may also be why we're reverting to all those gym-class classics like push-ups, jumping rope, and hula-hooping that are popping up everywhere from The Dr. Oz Show and Late Night with Jimmy Fallon to your fitness club's class schedule.)

This resurgence is also part of the natural cycle of food trends. For years, a steady stream of highly publicizedand celebrity-endorsedweight-loss books flooded the market. "But we're in a period of diet burnout," explains Brian Wansink, PhD, director of the Food and Brand Lab at Cornell University and author of Mindless Eating: Why We Eat More Than We Think. "Whenever a diet trend begins to die down because it doesn't deliver, there's renewed interest in these types of foods."

That's what is likely happening now, as waistline-watchers discover that gluten-free diets and exotic health foods like quinoa aren't automatically slimming. "I'm seeing people gain weight on gluten-free diets left and right," says Rachel Beller, RD, author of Eat to Lose, Eat to Win. "And things like quinoa and farro, while healthy, can be calorie bombs if you don't mind your portions."

The icing on the rice cakeUltimately, throwback diet foods are popular because they work. "I have two clients in their mid 30s, TV stars, who were trying sugar-free this and gluten-free that," says Christine Avanti, author of Skinny Chicks Eat Real Food and a celebrity nutritionist in Los Angeles. "When they threw out the gimmicky diet foods and busted out the regular cereal with skim milk, and the cottage cheese and baked potatoes, they started seeing results within a week."

After all, low-fat cottage cheese is a great source of satiating protein; research shows that eating grapefruit can reduce insulin levels, and aid in weight loss; and rice cakes, which are low-cal, give you a lot of bang for your bite. Even baked potatoesdiet terrorists during the low-carb crazecan help you lose.

These foods for the most part fall into two camps, explains Barbara Rolls, PhD, professor of nutritional sciences at Pennsylvania State University and author of The Ultimate Volumetrics Diet. "Things like Jell-O, cottage cheese, and cabbage have low calorie density, meaning they have few calories per ounce," Rolls says. "Foods such as air-popped popcorn and rice cakes have high volume, in that they are aerated and trick your mind into thinking you're eating more than you are. Both are effective for weight loss."

Some may even have pound-melting superpowers. "High-calcium foods (yogurt and cottage cheese) and high-fiber foods (like fruit and vegetables) may increase weight loss by either blocking the absorption of other calories or by enhancing metabolic efficiency," says weight control specialist Stephen Gullo, PhD, author of The Thin Commandments Diet. "We may not understand all the ways this works, but if you're losing weight, does it really matter?"